1. Field of the Invention
This invention relates to a method for manufacturing carbon nanotube structures applicable to devices which contain carbon nanotubes, functional materials, and other structural materials, to the carbon nanotube structures, and to carbon nanotube devices using them.
The invention can be extended to a wide variety of applications of carbon nanotubes.
2. Description of the Related Art
Fibrous carbons are generally called carbon fibers and conventionally, several kinds of methods for manufacturing carbon fibers having thickness of several μm or more in diameter used for structural materials have been studied. At present, among them, the method for manufacturing carbon fibers from PAN-based (polyacrylonitrile) and pitch-based materials is the most widely used.
The method is briefly described as such a method, by which materials spun out from the PAN-based, isotropic pitch-based, and mesophase pitch-based fibers are insolubilized, made flameproof, carbonized at 800 to 1400° C., and high-temperature treated at 1500 to 3000° C. Since the resulting carbon fibers not only have superior mechanical characteristics such as strength and elastic modulus but also are light weight, they are used for sporting goods, a heat-insulating material, a compound material for a structural material applicable to aerospace and automobile production.
Apart from this, the carbon nanotubes discovered recently are made of a tubular material with a thickness of 1 μm or smaller (in diameter). Ideally, a carbon face of a hexagon mesh forms a tube in parallel to an axis of the tube and plural tubes may be formed. It is theoretically estimated that the carbon nanotubes have either a metallic or semiconductor property depending on how carbon hexagon meshes are linked and the thickness of the tubes, allowing expectation that it will be a promising functional material.
Usually, to synthesize the carbon nanotubes, an arc discharge method is used and in addition, the methods including a laser evaporation method, a pyrolytic method, and a method using plasma have recently been studied. The carbon nanotubes recently developed are generally described below.
(Carbon Nanotube)
Finer than carbon fibers, the material with 1 μm or smaller of diameter is generally called carbon nanotubes and distinguished from the carbon fibers, although no clear line can be run between the both types of carbon fibers. By a narrow definition, the material, of which carbon faces with hexagon meshes are almost parallel to the axis of the tube, is called a carbon nanotube and even a variant of the carbon nanotube, around which amorphous carbon and metal or its catalyst surrounds, is included in the carbon nanotube. (Note that with respect to the present invention, this narrow definition is applied to the carbon nanotube.)
Usually, the narrowly-defined carbon nanotubes are further classified into two types: carbon nanotubes having a structure with a single hexagon mesh tube are called single wall nanotubes (hereafter, simply referred to as “SWNT”; the carbon nanotubes made of multi-layer hexagon mesh tubes are called multi-wall nanotubes (hereafter, simply referred to as “MWNT”). Type of carbon nanotubes may be determined depending on how to synthesize and the established conditions to some degree but production of purely one type of the carbon nanotubes has not yet been achieved.
The carbon fibers have larger diameters and incomplete cylindrical mesh structures parallel to the axes of the tubes. The carbon nanotubes produced by a vapor-phase pyrolysis method using a catalyst have a tubular mesh structure parallel to the axis of the tube in the vicinity of a center of the tube and in many cases, a large mount of carbon having a disordered structure surrounds it.
(Application of Carbon Nanotube)
Next, conventional applications of carbon nanotubes are described below. At present, no carbon nanotube-applied products have been yet put on the market but research and development activities are actively taken. Among of them, some typical examples are briefly described below.
(1) Electron Source
Since carbon nanotubes have sharp ends and electric conduction, in many studies, they have been treated as electron sources. W. A. deHeer et al. (Vol. 270, 1995, p1179) reported in “Science” that carbon nanotubes produced by the arc discharge method are prepared on a board with a filter after purification to use as electron sources. This report describes that a collection of carbon nanotubes was used for electron sources and an emission current of 100 mA or higher was stably gained from a 1 cm2 area by applying 700V of voltage.
Moreover, A. G. Rinzler et al. reported in “Science” (Vol. 269, 1995, p1550) that they attached one of the carbon nanotubes produced by the arc discharge method to an electrode and evaluated its characteristic, which proved that from a carbon nanotube, of which ends were closed, an emission current of about 1 nA and from a carbon nanotube, of which ends were open, an emission current of about 0.5 μA were gained, respectively when voltage of 75 V was applied.
(2) STM, AFM
H. Dai et al. reported the applications of carbon nanotubes to STM and AFM in “Nature” (384, 1996, p. 147). The carbon nanotubes used in this study were produced by the arc discharge method, of which ends were 5 nm-diameter SWNTs. It was said that since their tips were thin and flexible, they could be observed even at bottoms of gaps of a sample and the tips of the nanotube might hardly be crashed
(3) Hydrogen Storage Material
A. C. Dillon et al. reported in “Nature” (Vol. 386, 1997, p377 to 397) that the carbon nanotubes using SWNTs could store hydrogen molecules several times those for the carbon nanotubes made of a pitch-based material. Although a study about the applications has just begun, they are expected to be a promising material for hydrogen storage, for example, for hydrogen-fueled cars in the future.
At present, three types of methods are mainly used for manufacturing the carbon nanotubes mentioned above. Concretely, the methods include a method (the pyrolysis method using the catalyst) similar to the vapor-phase epitaxy method for manufacturing the carbon fibers, the arc discharge method, and the laser evaporation method. In addition to the three types of methods mentioned above, a plasma synthesis method and a solid reaction method are known.
Here, these typical three methods are briefly described below.
(1) The Pyrolysis Method Using the Catalyst
The method is almost the same as the vapor-phase epitaxy method for manufacturing the carbon fibers. The details of such a method have been described by C. E. SYNDER et al. in International Patent WO89/07163 (International Publication Number). It is indicated that ethylene and propane are introduced mixed with hydrogen as a material gas, as well as metal fine particles into a reaction vessel in their study and in addition to them, saturated hydrocarbon such as methane, ethane, propane, butane, hexane, and cyclohexane and oxygen such as acetone, methanol, and carbon monoxide may be used for the material gas.
The report described a preferable ratio of material gas and hydrogen of 1:20 to 20:1, recommended Fe or a mixture of Fe and Mo, Cr, Ce, or Mn as catalysts, and proposed a method, by which the catalyst was kept adhesive on a fumed alumina layer, as well. It is preferable that with regard to the reaction vessel, flow rates of the gas with hydrogen and the material gas with carbon are set to 100 sccm/inch and 200 sccm/inch, respectively at a temperature in a range of 550 to 850° C. and in this case, about 30 minutes to one hour after finely-divided particles are introduced, the carbon nanotubes begin to grow.
With respect to a shape of the resultant carbon nanotube, its diameter is about 3.5 to 75 nm and length is 5 to 1000 times the diameter. A mesh structure of carbon is parallel to an axis of the tube with less pyrolytic carbon adhered to an outer wall of the tube.
It was reported by H. Dai et al. (“Chemical Physics Letters” 260, 1996, p.471 to 475) that regardless of low efficiency of production, Mo was used as a catalytic nucleus and the material gas of carbon monoxide reacted at 1200° C., allowing SWNT to be produced.
(2) The Arc Discharge Method
The arc discharge method, which was first discovered by Iijima, is described in detail in “Nature” (Vol. 354, 1991, p 56 to 58). The arc discharge method is a simple method, by which direct current arc discharge is performed using carbon electrode rods in an atmosphere containing argon under about 13300 Pa (100 Torr). The carbon nanotubes grow with 5 to 20 nm of carbon particles in partial area on a surface of a negative electrode. The resultant carbon nanotubes have a layer structure, in which tubular carbon meshes with 4 to 30 nm of diameter and about 1 to 50 μm of length are overlapped; the mesh structure of carbon being helically formed in parallel with its axis.
Helical pitches depend on tubes or layers in the tube and for multilayer tubes, a distance between the layers is 0.34 nm, which is almost identical to a distance between graphite layers. The open ends of the tubes are also covered with a carbon interconnection.
Moreover, T. W. Ebbesen et al. reported a condition, in which a large amount of carbon nanotubes was produced by the arc discharge method in “Nature” (Vol. 358, 1992, p220 to 222). To be concrete, arc discharge of about 18 V and 100 A was generated in the condition, in which a 9 mm-diameter carbon rod for a cathode and a 6 mm-diameter carbon rod for an anode were used, respectively, which were oppositely disposed 1 mm apart from one another in a chamber, in the atmosphere containing helium under about 66500 Pa (500 Torr).
If a pressure lower than 66500 Pa (500 Torr) is applied, less carbon nanotubes are produced, while even if the pressure higher than 66500 Pa (500 Torr) is applied, the total amount of carbon nanotubes to be produced is small. In the optimal condition of 66500 Pa (500 Torr), a percentage of carbon nanotubes in a product reaches 75%. When an input power is varied or argon is contained in the atmosphere instead of helium, a yield of carbon nanotubes would become lower. Note that the carbon nanotubes are prone to gather in the vicinity of centers of the carbon rods.
(3) The Laser Evaporation Method
The laser evaporation method was reported by T. Guo et al. in “Chemical Physics Letters” (243, 1995, p. 49 to 54) and A. Thess et al. reported in “Science” (vol. 273, 1996, p. 483 to 487) that lope-like SWNTs were produced by the laser evaporation method. The method is generally described below.
After the carbon rods, of which surfaces Co and Ni are dispersed on, are put into a quartz tube and Ar (argon) is filled in the quartz tube under 66500 Pa (500 Torr), a whole tube is heated to about 1200° C. From an upstream end of the quartz tube, NdYAG laser is focused on the carbon rods to heat for evaporate. Then, on a downstream side of the quartz tube, carbon nanotubes are deposited. The method is a promising method for producing SWNTs selectively and has its own characteristics, for example, SWNTs are prone to gather into a rope like shape.
In the conventional carbon nanotube structures and manufacturing methods mentioned above, the resulting carbon nanotubes vary widely in both thickness and direction and immediately after they have been produced, the electrodes have not yet jointed the carbon nanotubes. This means that before the carbon nanotubes can be used, after synthesis, they must have been collected, purified, and formed into specific shapes depending on individual applications.
For example, since when an attempt was made to apply the carbon nanotubes to electric circuits, not only was it difficult to handle the carbon nanotubes because of their very fine sizes but also no method had been yet proposed for producing high-density wiring such as integrated circuits (ICs), the only thing subject to evaluation was that a single-structure fine element, which was produced by preparing fine electrodes, on which the carbon nanotubes were grown as shown in “Nature” (vol. 397, 1999, p. 673 to 675). In addition, it is preferable to build the carbon nanotubes in circuits effectively with no loss because they are very expensive.
The problems of difficult handling and expensiveness are large obstacles to actual application to devices.
As one of breakthroughs, electric signal processing simulating a mechanism of a brain, which is different from those of conditional electronic circuit devices, may be considered. Unlike conditional electric wirings, the carbon nanotubes provide multi-wirings as if they were neurons in brain, possibly allowing a non-Neumann type of processing mechanism, which is different from that of conventional computation to be implemented. Nevertheless, it has not yet been reported that a structure of carbon nanotubes and fibers was used to transmit and process signals.
It is estimated that a thin film can be manufactured from carbon nanotubes in a method for forming an organized structure of the carbon nanotubes in which the carbon nanotubes are well dispersed in the dispersion medium to prepare the liquid with the carbon nanotubes dispersed, the liquid is dropped on the planar board, and then the planar board is dried. So far, a practice has been performed that the liquid with the carbon nanotubes dispersed on is dropped on the planar board, leaving a trace amount of carbon nanotube lump by chance. These products have been treated as an infinitesimal residue, of which the amount was well within an expected range, instead of an established manufacturing method specific to the carbon nanotubes.
As known from the term “molecular self-assembly”, as the disperse medium dries gradually up from the liquid, which was prepared by the fine objects were dispersed in the dispersion medium, the fine objects may form a thin film, in which the fine objects are closely packed. However, in this case, the fine objects behave freely in the liquid without binding each other except for its aggregation. For this reason, when the fine objects are closely packed in the film during a drying process of dispersion-medium, contact between the fine objects each other is governed by only the aggregation exerted among the fine objects. This means that aggregates of fine objects are separated out and form a film together. This is the reason why a domain, in which the fine objects are closely packed, is prone to being formed of plural separate islands of the fine objects.
In the case of using conductive particles as fine objects, if they can be distributed into a network, even a small amount of fine objects allow the whole surface of the board to be conductive, although, as mentioned above, only by drying the dispersion medium from the liquid with the fine objects dispersed, the domain is prone to being stably formed of plural separate islands of the fine objects. This is the reason why, to make the planar board conductive, conductive particles has to be used by the amount, which can cover almost the whole surface of the planar board.
A problem of the liquid with the fine objects dispersed mentioned above, is essentially applicable to the case of the use of the carbon nanotubes instead. In the other words, since the carbon nanotube lumps, which were produced by dropping the liquid with the carbon nanotubes dispersed on the planar board and drying it up, usually were formed into plural separate islands and isolated within each of domains, electrical and/or magnetic connectivity among them was broken. This has been an obstacle to forming a useful network for a carbon nanotube structure. For this reason, simply to make the whole surface of the planar board conductive, the carbon nanotubes had to be used by the amount, which could cover almost the whole surface of the planar board. In this case, since a remarkably large amount of carbon nanotubes are required, there is no merit found in the use of carbon nanotubes.